It is known how to make observable the pressure variations which take place, by means of a spring tensioned diaphragm; such indicating devices are applied and used in a number of manners in several technical domains.
One possibile application of such devices is to indicate the degree of fullness of the household or industrial vacuum cleaners. The invention concerns particularly such an indicating device, but it could, of course, be used in other comparable applications, for example, whenever a pressure modification attaining a certain threshold value has to be indicated externally, or when pressure modifications are used for controlling or regulating purposes and for the introduction of other working functions.
The indication of the fullness of dust bags or vacuum cleaners in general which do not include a bag-like dust catching system, is known to use the vacuum pressure which increases as the vacuum cleaner fills with dust, by means of an indicating element which is located in a pressure tight conduit and is pushed against the holding force of a spring from its initial position under the influence of the acting pressure. Such displacement of the prefereably colored indicating element can be observed by the appliance's operator through an inspection window so that he can proceed to empty the vacuum cleaner when the indicating element has attained a certain position corresponding to a certain maximum (allowable) load, beyond which the vacuum cleaner operates only uneconomically.
It is also known how to replace such a mechanical indication by an electrical contact circuit. In this case a diaphragm is also used, and is located and carried in an appropriate manner. This diaphragm closes a space which communicates with the vacuum pressure, or in general with the pressure variations generating system. The diaphragm is essentially flexible but is however held taut by an appropriate spring in such a way, that at a given displacement of the diaphragm corresponding to pressure variations, a contact element carried by the diaphragm comes in contact with a second, stationarily positioned contact element. In this manner an electric circuit is closed, which produces an indication or a different switching action. The drawback of this known system consists in the fact that an adjustment of the device to a desirable predetermined threshold pressure value is relatively complicated, since one has to modify the tension produced by the spring on the pretensioned diaphragm in such a way that the contact takes place precisely at the predetermined threshold pressure value. It can be done by the modification of the application point of the spring, which, however, demands that the system, after the finished final assembly of other components, has to be dismantled again; a certain vacuum pressure value has then to be produced and the adjustment has to be made to correspond to that value.
Such an adjustment is necessary, since the pressure data of the substantially identical appliances differ from each other within certain value limits.